Basic Data Types

 

Rodney Dyer, PhD

Assignment Operator

In R, values can be assigned to variables using the -> operator.

  • Variable names should be informative
    • Must start with letter
    • Can include numbers, underscore, periods
    • No spaces
    • Case sensitive.
  • Assignment is directional -> and <- work.
  • Use names that won’t make your future self hate your present self

Basic Data Types

There are several different data types in R, each of which represents some identifiable component in our analysis mileau. For this quick lecture, we are going to focus on five of the most common data types.

  • Missing Data: The absence of anything.
  • Numerical Data: Representing components as a measurable quantity.
  • Character Data: Representations in textual values.
  • Logical Data: TRUE or FALSE values.
  • Dates: Dates and Time

No Data

Missing Data

The most basic type of data is the kind that does not exist. In R, we represent this using the NA data type.

x <- NA
x
[1] NA

Numerical Data

Numerical Data

Numerical data is the most common type of data that we will all be using. In R, most computations use this type.

 

  • Counts, measurements, etc.
  • Decimal or Integer
  • Real and Imaginary

Example of assignment

42 + 24
[1] 66

Examples

Here are some examples of assignment values, in both directions. I find it helpful to read out the code directly

pop1.x ‘is assigned the value of’ 23

 

pop1.x <- 23
pop1.y <- 55
56 -> pop2.x
63 -> pop2.y

Viewing The Values in Variables

And we can see the values stored in those variables by simply typing the names of the variables.

 

pop1.x 
[1] 23

 

pop1.y
[1] 55

You can also view the value in a variable by looking in the Environment tab in RStudio

Operators

Most common operators are available and work just the way that you think. The order of operations is still the same as well, you can use parentheses to order things if necessary.

&nbps;

distance <- sqrt( (pop1.x - pop2.x)^2 + (pop1.y - pop2.y)^2  )
distance
[1] 33.95585

Named Functions

As in the previous example, the mathematical function sqrt() performs a square root operator.

  • Some operators are written like functions. 
    function_name + opening parenthesis + arguments + closing parenthesis
  • Others are combinations of existing glyphs surrounded by percentages. 
    percent sign + operator + percent sign
  • There is just not enough keys (and associated glyphs) on the keyboard to handle all the potential types of operators.

Order of Operations (Top to Bottom)

You can find this list by ?Syntax and the help system will show you the following table.

Operator Description
:: ::: access variables in a namespace
$ @ component / slot extraction
[ [[ indexing
^ exponentiation (right to left)
- + unary minus and plus
: sequence operator
%any% special operators (including %% and %/%)
* / multiply, divide
+ - (binary) add, subtract
< > <= >= == != ordering and comparison
! negation
& && and
~ as in formula
-> ->> rightwards assignment
<- <<- assignment (right to left)
= assignment (right to left)
? help (unary and binary)

Data Classes

Each data type in R can be asked what class of variable it points to.

 

x <- 42
class( x )
[1] "numeric"

This class type is a data type identifier.

Introspection

We can also ask a variable if it is a particular type of variable. In R, the convention is to create a function that has the form of is.datatype() that will determine if the variable passed to the function is that datatype.

 

The values they return are TRUE if it is

is.numeric( pop1.x )
[1] TRUE

or FALSE if it is not

is.numeric( "Dyer is my favorite professor")
[1] FALSE

Uses for Introspection

It is not common to use introspection UNTIL your code does not work for some reason. Then it is really important to try to figure out what the problem is and if the data we are looking at are really the type we are thinking they are.

x <- "42"
x + 23
Error in x + 23: non-numeric argument to binary operator

 

class(x)
[1] "character"
class( 23 )
[1] "numeric"

Coercion

There are times (like in the last slide) that we’d like to coerce a variable of one type into another type. In this case, we’d like to take the character value pointed to by x and coerce it into a numeric type so that we can perform that addition operator.

Coercion

We have a coercion operator that has the form as.datatype(). If possible, it will convert the passed variable into the other datatype

as.numeric( x ) + 23 
[1] 65

If not, it will give return a missing value (NA)

as.numeric( "Bob" )
[1] NA

Character Data

Character

All textual data is of type character and is defined as zero or more glyphs surrounded by either single or double quotes.

 

firstName <- "Rodney"
firstName
[1] "Rodney"
class( firstName )
[1] "character"

Introspection

As for numeric data types, character also have an introspection operator.

 

is.character( firstName )
[1] TRUE

 

is.character( 23 )
[1] FALSE

 

is.character( "23" )
[1] TRUE

Coercion Operators

The character data type is the most basal type and as such, any type of data can be coerced into its character representation.

 

y <- as.character( pi )
y
[1] "3.14159265358979"

Combination Operators

There are many times that it helpful to combine more than one variable containing character data into a single variable.

 

firstName
[1] "Rodney"
lastName <- "Dyer"

Using the paste() function.

fullName <- paste( firstName, lastName)
fullName
[1] "Rodney Dyer"
class( fullName )
[1] "character"

Logical Data

Logical Data

Logical data consists of two mutually exclusive states: TRUE or FALSE

 

dyer_has_good_jokes <- TRUE
dyer_has_good_jokes
[1] TRUE

Operators on Logical Types

There are 3 primary logical operators that can be used on logical types; one unary and two binary.

 

Unary Operator

The negation operator

!dyer_has_good_jokes
[1] FALSE
!(!dyer_has_good_jokes)
[1] TRUE

The Binary Operators

These operators require two separate items to operate on, one to the left and another to the right of the operator.

 

The OR operator

TRUE | FALSE
[1] TRUE

The AND operator

TRUE & FALSE
[1] FALSE

Introspection

Logical types have an introspection operator.

 

is.logical( dyer_has_good_jokes )
[1] TRUE

Coercion

Coercion of something else to a Logical is more case-specific.

 

From character data.

as.logical( "TRUE" )
[1] TRUE

 

as.logical( "FALSE" )
[1] FALSE

Incompatible Coercion

Other character types result in NA (missing data).

as.logical( "Bob" )
[1] NA

Coercion

Coercion of something else to a Logical is more case-specific.

From numeric data:
- Values of 0 are FALSE
- Non-zero values are TRUE

 

as.logical(0)
[1] FALSE
as.logical( 323 )
[1] TRUE

Dates

What are Dates & Times

When we read textual representations of dates and times, we intuitively understand their representations. So we know what

  • February 3, 1996
  • Tomorrow @ Noon
  • Next Thursday afternoon

Dates & Time in R

But for R, we need to be able to tell the computer what these sequences of characters mean in a way that they can understand that something like 03/01/22 means either March 1, 2022 (if you are in the US) or January 3, 2022 (if you are in Europe) and that intrinsic relationships between what a March and a January really mean.

Computer Time

Computers keep time as the number of seconds since the epoch, which is defined as midnight on 1 January 1970.

 

You can have R ask your computer what the time is and it will give it to you in the format that your locale is set to (probably US-en).

Sys.time()
[1] "2023-11-28 08:53:32 EST"

Unix Epoch Time

But this is just a representation of the time for your geospatial position. Internally, it

unclass( Sys.time() )
[1] 1701179612

Helpful Library

There are a few R packages that deal with various issue about time as well as some built-in functionality. I personally prefer to use the library lubridate because it is part of tidyverse and it is very easy to work with.

Installing the library if you do not have it (this only need to be done once on your computer).

install.packages("lubridate")

 

Then tell R you would like to use it.

library( lubridate )

Creating Dates

To create dates, we typically start with character representations of date and time objects.

sep1 <- "9/1/22"
class( sep1 )
[1] "character"
sep1
[1] "9/1/22"

Creating Dates

So this is a month/day/year format. We can use the function mdy() and pass it the character representation of the date.

today <- mdy( sep1 )
class( today )
[1] "Date"
today
[1] "2022-09-01"

Actual Today

There are also a few built-in functions that we can use to help reduce our need to type.

today <- today()
class(today)
[1] "Date"
today
[1] "2023-11-28"

Date Operators

Things that are intrinsically “Date” objects have operators that can be used. Examples include adding/subtracting days, determine day of the week, or the Julian day.

 

today - 1
[1] "2023-11-27"

 

weekdays( today )
[1] "Tuesday"

 

yday( today )
[1] 332

Interval Conversions

By default, many operations on date objects are based upon days.

 

hired <- mdy( "08/25/2004")
days <- today - hired 
days 
Time difference of 7034 days

Incremenatlly Building Dates & Times

You can add arbitrary components to dates using these helper functions

today + months(1)+ weeks(1) + days(6) + hours(4) - minutes(35) + seconds(15) 
[1] "2024-01-10 03:25:15 UTC"

Making Time

Just like making a date, we can make time elements.

 

classtime <- mdy_hms("09/01/2022 12:30 PM")
classtime
[1] "2020-09-01 22:12:30 UTC"

Setting a Time Zone

The time zone is best set when you create the date/time object.

today <- mdy_hm("09/01/2022 12:30 PM", tz="US/Eastern")
today
[1] "2022-09-01 12:30:00 EDT"

Time Classes

However, they MUST be compatible (these are not because hired has to timezone).

hired <- ymd("2004/08/23", tz="US/Eastern")
howLong <- today - hired
class( howLong )
[1] "difftime"
howLong
Time difference of 6583.521 days

Examples

today() - mdy("10/14/1969") -> daysAlive
daysAlive
Time difference of 19768 days

Dyer’s Half Life

daysAlive/2 + mdy("10/14/1969")
[1] "1996-11-05"

When Is Dyer 20,000 Days Old

mdy("10/14/1969") + days(20000) -> day20K

 

On Wednesday, July 17, 2024 Dyer will be 20,000 days old.

More Date Operations

There are a lot more operations you can do with date and time objects. I recommend you look at the Cheatsheet in the repository for the lubridate library.

Help for Date & Time Conversions

R: Date-time Conversion Functions to and from Character
strptime R Documentation

Date-time Conversion Functions to and from Character

Description

Functions to convert between character representations and objects of classes "POSIXlt" and "POSIXct" representing calendar dates and times.

Usage

## S3 method for class 'POSIXct'
format(x, format = "", tz = "", usetz = FALSE, ...)
## S3 method for class 'POSIXlt'
format(x, format = "", usetz = FALSE,
       digits = getOption("digits.secs"), ...)

## S3 method for class 'POSIXt'
as.character(x, digits = if(inherits(x, "POSIXlt")) 14L else 6L,
             OutDec = ".", ...)

strftime(x, format = "", tz = "", usetz = FALSE, ...)
strptime(x, format, tz = "")

Arguments

x

An object to be converted: a character vector for strptime, an object which can be converted to "POSIXlt" for strftime.
tz

A character string specifying the time zone to be used for the conversion. System-specific (see as.POSIXlt), but "" is the current time zone, and "GMT" is UTC. Invalid values are most commonly treated as UTC, on some platforms with a warning.
format

A character string. The default for the format methods is "%Y-%m-%d %H:%M:%S" if any element has a time component which is not midnight, and "%Y-%m-%d" otherwise. If options("digits.secs") is set, up to the specified number of digits will be printed for seconds.
...

Further arguments to be passed from or to other methods.
usetz

logical. Should the time zone abbreviation be appended to the output? This is used in printing times, and more reliable than using "%Z".
digits

integer determining the format()ing of seconds when needed. Note that the defaults for format() and as.character() differ on purpose, as.character() giving close to full accuracy as it does for numbers.
OutDec

a 1-character string specifying the decimal point to be used; the default is not getOption("OutDec") on purpose.

Details

The format and as.character methods and strftime convert objects from the classes "POSIXlt" and "POSIXct" to character vectors.

strptime converts character vectors to class "POSIXlt": its input x is first converted by as.character. Each input string is processed as far as necessary for the format specified: any trailing characters are ignored.

strftime is a wrapper for format.POSIXlt, and it and format.POSIXct first convert to class "POSIXlt" by calling as.POSIXlt (so they also work for class "Date"). Note that only that conversion depends on the time zone. Since R version 4.2.0, as.POSIXlt() conversion now treats the non-finite numeric -Inf, Inf, NA and NaN differently (where previously all were treated as NA). Also the format() method for POSIXlt now treats these different non-finite times and dates analogously to type double.

The usual vector re-cycling rules are applied to x and format so the answer will be of length of the longer of these vectors.

Locale-specific conversions to and from character strings are used where appropriate and available. This affects the names of the days and months, the AM/PM indicator (if used) and the separators in output formats such as %x and %X, via the setting of the LC_TIME locale category. The ‘current locale’ of the descriptions might mean the locale in use at the start of the R session or when these functions are first used. (For input, the locale-specific conversions can be changed by calling Sys.setlocale with category LC_TIME (or LC_ALL). For output, what happens depends on the OS but usually works.)

The details of the formats are platform-specific, but the following are likely to be widely available: most are defined by the POSIX standard. A conversion specification is introduced by %, usually followed by a single letter or O or E and then a single letter. Any character in the format string not part of a conversion specification is interpreted literally (and %% gives %). Widely implemented conversion specifications include

%a

Abbreviated weekday name in the current locale on this platform. (Also matches full name on input: in some locales there are no abbreviations of names.)

%A

Full weekday name in the current locale. (Also matches abbreviated name on input.)

%b

Abbreviated month name in the current locale on this platform. (Also matches full name on input: in some locales there are no abbreviations of names.)

%B

Full month name in the current locale. (Also matches abbreviated name on input.)

%c

Date and time. Locale-specific on output, "%a %b %e %H:%M:%S %Y" on input.

%C

Century (00–99): the integer part of the year divided by 100.

%d

Day of the month as decimal number (01–31).

%D

Date format such as %m/%d/%y: the C99 standard says it should be that exact format (but not all OSes comply).

%e

Day of the month as decimal number (1–31), with a leading space for a single-digit number.

%F

Equivalent to %Y-%m-%d (the ISO 8601 date format).

%g

The last two digits of the week-based year (see %V). (Accepted but ignored on input.)

%G

The week-based year (see %V) as a decimal number. (Accepted but ignored on input.)

%h

Equivalent to %b.

%H

Hours as decimal number (00–23). As a special exception strings such as ‘⁠24:00:00⁠’ are accepted for input, since ISO 8601 allows these.

%I

Hours as decimal number (01–12).

%j

Day of year as decimal number (001–366): For input, 366 is only valid in a leap year.

%m

Month as decimal number (01–12).

%M

Minute as decimal number (00–59).

%n

Newline on output, arbitrary whitespace on input.

%p

AM/PM indicator in the locale. Used in conjunction with %I and not with %H. An empty string in some locales (for example on some OSes, non-English European locales including Russia). The behaviour is undefined if used for input in such a locale.

Some platforms accept %P for output, which uses a lower-case version (%p may also use lower case): others will output P.

%r

For output, the 12-hour clock time (using the locale's AM or PM): only defined in some locales, and on some OSes misleading in locales which do not define an AM/PM indicator. For input, equivalent to %I:%M:%S %p.

%R

Equivalent to %H:%M.

%S

Second as integer (00–61), allowing for up to two leap-seconds (but POSIX-compliant implementations will ignore leap seconds).

%t

Tab on output, arbitrary whitespace on input.

%T

Equivalent to %H:%M:%S.

%u

Weekday as a decimal number (1–7, Monday is 1).

%U

Week of the year as decimal number (00–53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1). The US convention.

%V

Week of the year as decimal number (01–53) as defined in ISO 8601. If the week (starting on Monday) containing 1 January has four or more days in the new year, then it is considered week 1. Otherwise, it is the last week of the previous year, and the next week is week 1. (Accepted but ignored on input.)

%w

Weekday as decimal number (0–6, Sunday is 0).

%W

Week of the year as decimal number (00–53) using Monday as the first day of week (and typically with the first Monday of the year as day 1 of week 1). The UK convention.

%x

Date. Locale-specific on output, "%y/%m/%d" on input.

%X

Time. Locale-specific on output, "%H:%M:%S" on input.

%y

Year without century (00–99). On input, values 00 to 68 are prefixed by 20 and 69 to 99 by 19 – that is the behaviour specified by the 2018 POSIX standard, but it does also say ‘it is expected that in a future version the default century inferred from a 2-digit year will change’.

%Y

Year with century. Note that whereas there was no zero in the original Gregorian calendar, ISO 8601:2004 defines it to be valid (interpreted as 1BC): see https://en.wikipedia.org/wiki/0_(year). However, the standards also say that years before 1582 in its calendar should only be used with agreement of the parties involved.

For input, only years 0:9999 are accepted.

%z

Signed offset in hours and minutes from UTC, so -0800 is 8 hours behind UTC. (Standard only for output. For input R currently supports it on all platforms – values from -1400 to +1400 are accepted.)

%Z

(Output only.) Time zone abbreviation as a character string (empty if not available). This may not be reliable when a time zone has changed abbreviations over the years.

Where leading zeros are shown they will be used on output but are optional on input. Names are matched case-insensitively on input: whether they are capitalized on output depends on the platform and the locale. Note that abbreviated names are platform-specific (although the standards specify that in the ‘⁠C⁠’ locale they must be the first three letters of the capitalized English name: this convention is widely used in English-language locales but for example the French month abbreviations are not the same on any two of Linux, macOS, Solaris and Windows). Knowing what the abbreviations are is essential if you wish to use %a, %b or %h as part of an input format: see the examples for how to check.

When %z or %Z is used for output with an object with an assigned time zone an attempt is made to use the values for that time zone — but it is not guaranteed to succeed.

The definition of ‘whitespace’ for %n and %t is platform-dependent: for most it does not include non-breaking spaces.

Not in the standards and less widely implemented are

%k

The 24-hour clock time with single digits preceded by a blank.

%l

The 12-hour clock time with single digits preceded by a blank.

%s

(Output only.) The number of seconds since the epoch.

%+

(Output only.) Similar to %c, often "%a %b %e %H:%M:%S %Z %Y". May depend on the locale.

For output there are also %O[dHImMUVwWy] which may emit numbers in an alternative locale-dependent format (e.g., roman numerals), and %E[cCyYxX] which can use an alternative ‘era’ (e.g., a different religious calendar). Which of these are supported is OS-dependent. These are accepted for input, but with the standard interpretation.

Specific to R is %OSn, which for output gives the seconds truncated to 0 <= n <= 6 decimal places (and if %OS is not followed by a digit, it uses the setting of getOption("digits.secs"), or if that is unset, n = 0). Further, for strptime %OS will input seconds including fractional seconds. Note that %S does not read fractional parts on output.

The behaviour of other conversion specifications (and even if other character sequences commencing with % are conversion specifications) is system-specific. Some systems document that the use of multi-byte characters in format is unsupported: UTF-8 locales are unlikely to cause a problem.

Value

The format methods and strftime return character vectors representing the time. NA times are returned as NA_character_.

strptime turns character representations into an object of class "POSIXlt". The time zone is used to set the isdst component and to set the "tzone" attribute if tz != "". If the specified time is invalid (for example ‘⁠"2010-02-30 08:00"⁠’) all the components of the result are NA. (NB: this does means exactly what it says – if it is an invalid time, not just a time that does not exist in some time zone.)

Printing years

Everyone agrees that years from 1000 to 9999 should be printed with 4 digits, but the standards do not define what is to be done outside that range. For years 0 to 999 most OSes pad with zeros or spaces to 4 characters, but Linux/glibc outputs just the number.

OS facilities will probably not print years before 1 CE (aka 1 AD) ‘correctly’ (they tend to assume the existence of a year 0: see https://en.wikipedia.org/wiki/0_(year), and some OSes get them completely wrong). Common formats are -45 and -045.

Years after 9999 and before -999 are normally printed with five or more characters.

Some platforms support modifiers from POSIX 2008 (and others). On Linux/glibc the format "%04Y" assures a minimum of four characters and zero-padding (the default is no padding). The internal code (as used on Windows and by default on macOS) uses zero-padding by default (this can be controlled by environment variable R_PAD_YEARS_BY_ZERO). On those platforms, formats %04Y, %_4Y and %_Y can be used for zero, space and no padding respectively. (On macOS, the native code (not the default) supports none of these and uses zero-padding to 4 digits.)

Time zone offsets

Offsets from GMT (also known as UTC) are part of the conversion between timezones and to/from class "POSIXct", but cause difficulties as they are often computed incorrectly.

They conventionally have the opposite sign from time-zone specifications (see Sys.timezone): positive values are East of the meridian. Although there have been time zones with offsets like +00:09:21 (Paris in 1900), and -00:44:30 (Liberia until 1972), offsets are usually treated as whole numbers of minutes, and are most often seen in RFC 5322 email headers in forms like -0800 (e.g., used on the Pacific coast of the USA in winter).

Format %z can be used for input or output: it is a character string, conventionally plus or minus followed by two digits for hours and two for minutes: the standards say that an empty string should be output if the offset is undetermined, but some systems use +0000 or the offsets for the time zone in use for the current year. (On some platforms this works better after conversion to "POSIXct". Some platforms only recognize hour or half-hour offsets for output.)

Using %z for input makes most sense with tz = "UTC".

Sources

Input uses the POSIX function strptime and output the C99 function strftime.

However, not all OSes (notably Windows) provided strptime and many issues were found for those which did, so since 2000 R has used a fork of code from ‘⁠glibc⁠’. The forked code uses the system's strftime to find the locale-specific day and month names and any AM/PM indicator.

On some platforms (including Windows and by default on macOS) the system's strftime is replaced (along with most of the rest of the C-level datetime code) by code modified from IANA's ‘⁠tzcode⁠’ distribution (https://www.iana.org/time-zones).

Note that as strftime is used for output (and not wcsftime), argument format is translated if necessary to the session encoding.

Note

The default formats follow the rules of the ISO 8601 international standard which expresses a day as "2001-02-28" and a time as "14:01:02" using leading zeroes as here. (The ISO form uses no space, possibly ‘⁠T⁠’, to separate dates and times: R uses a space by default.)

For strptime the input string need not specify the date completely: it is assumed that unspecified seconds, minutes or hours are zero, and an unspecified year, month or day is the current one. (However, if a month is specified, the day of that month has to be specified by %d or %e since the current day of the month need not be valid for the specified month.) Some components may be returned as NA (but an unknown tzone component is represented by an empty string).

If the time zone specified is invalid on your system, what happens is system-specific but it will probably be ignored.

Remember that in most time zones some times do not occur and some occur twice because of transitions to/from ‘daylight saving’ (also known as ‘summer’) time. strptime does not validate such times (it does not assume a specific time zone), but conversion by as.POSIXct will do so. Conversion by strftime and formatting/printing uses OS facilities and may return nonsensical results for non-existent times at DST transitions.

In a C locale %c is required to be "%a %b %e %H:%M:%S %Y". As Windows does not comply (and uses a date format not understood outside N. America), that format is used by R on Windows in all locales.

There is a limit of 2048 bytes on each string produced by strftime and the format methods. As from R 4.3.0 attempting to exceed this is an error (previous versions silently truncated at 255 bytes).

References

International Organization for Standardization (2004, 2000, ...) ‘ISO 8601. Data elements and interchange formats – Information interchange – Representation of dates and times.’, slightly updated to International Organization for Standardization (2019) ‘ISO 8601-1:2019. Date and time – Representations for information interchange – Part 1: Basic rules’, and further amended in 2022. For links to versions available on-line see (at the time of writing) https://dotat.at/tmp/ISO_8601-2004_E.pdf and https://www.qsl.net/g1smd/isopdf.htm; for information on the current official version, see https://www.iso.org/iso/iso8601 and https://en.wikipedia.org/wiki/ISO_8601.

The POSIX 1003.1 standard, which is in some respects stricter than ISO 8601.

See Also

DateTimeClasses for details of the date-time classes; locales to query or set a locale.

Your system's help page on strftime to see how to specify their formats. (On some systems, including Windows, strftime is replaced by more comprehensive internal code.)

Examples


## locale-specific version of date()
format(Sys.time(), "%a %b %d %X %Y %Z")

## time to sub-second accuracy (if supported by the OS)
format(Sys.time(), "%H:%M:%OS3")

## read in date info in format 'ddmmmyyyy'
## This will give NA(s) in some non-English locales; setting the C locale
## as in the commented lines will overcome this on most systems.
## lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
x <- c("1jan1960", "2jan1960", "31mar1960", "30jul1960")
z <- strptime(x, "%d%b%Y")
## Sys.setlocale("LC_TIME", lct)
z
(chz <- as.character(z)) # same w/o TZ
## *here* (but not in general), the same as format():
stopifnot(exprs = {
     identical(chz, format(z))
     grepl("^1960-0[137]-[03][012]$", chz[!is.na(z)])
})

## read in date/time info in format 'm/d/y h:m:s'
dates <- c("02/27/92", "02/27/92", "01/14/92", "02/28/92", "02/01/92")
times <- c("23:03:20", "22:29:56", "01:03:30", "18:21:03", "16:56:26")
x <- paste(dates, times)
z2 <- strptime(x, "%m/%d/%y %H:%M:%S")
z2 
## *here* (but not in general), the same as format():
stopifnot(identical(format(z2), as.character(z2)))

## time with fractional seconds
z3 <- strptime("20/2/06 11:16:16.683", "%d/%m/%y %H:%M:%OS") 
z3 # prints without fractional seconds by default, digits.sec = NULL ("= 0")
op <- options(digits.secs = 3)
z3 # shows the 3 extra digits
as.character(z3) # ditto
options(op)

## time zone names are not portable, but 'EST5EDT' comes pretty close.
z4 <- strptime(c("2006-01-08 10:07:52", "2006-08-07 19:33:02"),
               "%Y-%m-%d %H:%M:%S", tz = "EST5EDT")
z4 
attr(z4, "tzone")
as.character(z4)
z4$sec[2] <- pi # "very" fractional seconds
as.character(z4) # shows full precision
format(z4) # no fractional sec
format(z4, digits=8) # shows only 6  (hard-wired maximum)
format(z4, digits=4)


## An RFC 5322 header (Eastern Canada, during DST)
## In a non-English locale the commented lines may be needed.

## prev <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
strptime("Tue, 23 Mar 2010 14:36:38 -0400", "%a, %d %b %Y %H:%M:%S %z")
## Sys.setlocale("LC_TIME", prev)

## Make sure you know what the abbreviated names are for you if you wish
## to use them for input (they are matched case-insensitively):
format(s1 <- seq.Date(as.Date('1978-01-01'), by = 'day',   len =  7), "%a")
format(s2 <- seq.Date(as.Date('2000-01-01'), by = 'month', len = 12), "%b")

## Non-finite date-times :
format(as.POSIXct(Inf)) # "Inf"  (was  NA  in R <= 4.1.x)
format(as.POSIXlt(c(-Inf,Inf,NaN,NA))) # were all NA

This is t

Questions

If you have any questions, please feel free to either post them to the Canvas discussion board for the class, or drop me an email.

Peter Sellers looking bored

Dyerlab